The proposed studies are directed toward evaluation of megakaryocytopoiesis, kinetic properties of megakaryocyte precursor cells, and homeostatic control of platelet production. The mechanisms underlying macromegakaryocytosis in two strains of genetically dyshemopoietic mice will be analyzed. Three animal models will be studied in which stimulation of platelet production appears to occur even though platelet counts are normal. The effects of inhibition of DNA synthesis on megakaryocytopoiesis and platelet production will be evaluated. A bioassay system for thrombopoietin will be evaluated using plasma fractionation.